JP3822711B2 - Alloyed hot-dip galvanized steel sheet - Google Patents

Description

【００４９】
【発明の効果】
本発明の合金化溶融亜鉛めっき鋼板は、加工成形前は強度レベルが３００Ｍｐａ級、４００Ｍｐａ級、５００Ｍｐａ級、６００Ｍｐａ級、あるいは７００Ｍｐａ級で比較的軟質の高強度鋼でプレス成形等の加工成形がしやすく、そして、プレス成形等の成形加工後には比較的低温での短時間熱処理を行うことで、引張強さ又は硬さが上昇し部材や部品の変形強度を高めるか、あるいは剛性を高めることが可能な素鋼板であるため、建設部材、機械構造用部品、自動車の構造用部品など、構造上の強度、特に変形時の強度及び又は剛性が必要とされる箇所に適用される。
【図面の簡単な説明】
【図１】本発明鋼板と従来鋼板の応力―歪み曲線を説明する模式図である。
【図２】本発明鋼板にプレス成形で付与する歪み量と成形後熱処理での引張強さ上昇量の関係を示す図である。
【図３】成形後熱処理温度と熱処理後の引張強さ上昇量との関係を示す図である。
【図４】ハット型のプレス成形品の形状を示す模式図である。
【符号の説明】
Ａ 壁部
Ｂ 平坦部[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is an alloyed hot dip galvanizing suitable for being applied to places where structural strength, particularly strength and / or rigidity during deformation, is required, such as construction members, machine structural parts, and automobile structural parts. In particular, the present invention relates to an alloyed hot-dip galvanized steel sheet used as a base steel sheet of a formed body that is subjected to heat treatment for increasing strength in a predetermined temperature range after work forming by a press or the like.
[0002]
[Prior art]
When manufacturing a press-formed body made of a thin steel plate, before press forming, it is soft and easy to press-form, and as a method of hardening after press forming to increase the strength of the part, there is a method of baking at less than 200 ° C. A BH steel sheet has been developed as a steel sheet for paint baking.
[0003]
For example, in Nb-added steels as disclosed in JP-A-55-141526 and JP-A-55-141555, Nb is added in accordance with the C, N, and Al contents in the steel and at Nb / (at%). The method of adjusting the solid solution C and solid solution N in the steel sheet by restricting the solid solution C + solid solution N) within a specific range and controlling the cooling rate after annealing, or Japanese Patent Publication No. 61-45689 As disclosed in Japanese Laid-Open Patent Publication No. 2004-260, it is disclosed that bake hardenability is improved by combined addition of Ti and Nb.
[0004]
However, since the steel plate as described above is made of a material having excellent deep drawability, the strength of the steel plate is low and is not necessarily sufficient as a structural material.
[0005]
Furthermore, as disclosed in JP-A-5-25549, it is disclosed that bake hardenability is improved by adding W, Cr, or Mo alone or in combination to steel. However, the increase in strength due to bake hardening is due to the use of solute C and solute N contained in the steel sheet, for example, as schematically shown in the stress-strain curve of the broken BH steel sheet in FIG. Only the yield strength of the material is increased (stress increase due to BH in FIG. 1), and the tensile strength is not increased.
[0006]
For this reason, there is only an effect of increasing the stress of starting deformation of the component, and the effect of increasing the stress required for deformation (hereinafter referred to as deformation strength characteristics) throughout the entire deformation from the start of deformation of the component to the end of deformation is not necessarily sufficient. .
[0007]
As a curing method other than press baking of the press-molded body, there is a method by soft nitriding after press molding. For example, a method in which a nitride-forming element such as Cr, Al, V or the like is included in steel so that the strength is increased by nitriding as in JP-A-2-80539, or a nitriding treatment as in JP-A-3-122255. A method of increasing the hardness of a member by precipitation-hardening Cu using the heat of the above is disclosed. However, these methods have the drawback that when the normal galvanizing is used to increase the corrosion resistance and the heating temperature is high, the plating layer is evaporated and it is difficult to obtain a product having good corrosion resistance.
[0008]
[Problems to be solved by the invention]
In order to solve the above-described problems, the present invention provides a strength level of 300 Mpa class, 400 Mpa class, 500 Mpa class, 600 Mpa class, 700 Mpa class, and relatively soft high strength steel before press molding. Is it easy to process and perform short-time heat treatment at a relatively low temperature for the purpose of increasing strength after press processing, etc., to increase tensile strength or hardness and increase deformation strength of members and parts? Alternatively, it is an object of the present invention to provide an alloyed hot-dip galvanized steel sheet as a base steel sheet capable of increasing rigidity.
[0009]
[Means for Solving the Problems]
The inventors of the present invention have provided a process for forming shapes of various molding materials and parts made of alloyed hot dip galvanized thin steel sheets, a heat treatment method for curing members and parts by heat treatment, and the steel sheets. We conducted intensive research on the deformation strength characteristics of press-formed bodies as parts made of As a result, by adding a suitable amount of C, W, Cr, and Mo in the steel composition, and further by forming an alloyed hot-dip galvanized steel sheet that specifies the microstructure of the steel, the corrosion resistance is not impaired. It was newly discovered that high strength increase (or hardness increase) can be obtained by short-time heat treatment in a temperature range of 200 to 450 ° C.
[0010]
The present invention has been accomplished based on this discovery, and the gist of the invention is as follows:
(1) C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or more of W, Cr and Mo in a total amount of 0.05 to 3.0%, the balance being iron and inevitable impurities, and the microstructure of the steel is ferrite An alloyed hot-dip galvanized steel sheet having post-forming strength increasing heat treatment performance, characterized by comprising:
[0011]
(2) As a steel composition, in further weight percent,
Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%,
V: 0.005 to 0.1%,
The alloyed hot-dip galvanized steel sheet having strength-increasing heat treatment performance after forming as described in (1) above, wherein one or more of the above are contained.
[0012]
(3) C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or more of W, Cr, and Mo, the total amount is 0.05 to 3.0% ,
Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%,
V: 0.005 to 0.1%,
An alloying having strength-heat-treating performance after forming, characterized in that the steel contains one or more of the following, the balance being iron and inevitable impurities, and the microstructure of the steel is mainly composed of ferrite Hot dip galvanized steel sheet.
[0014]
( 4 ) The alloyed hot-dip galvanized steel sheet having post-forming strength increasing heat treatment performance as described in (3) above, wherein the ferrite structure fraction is 60% or more.
[0015]
( 5 ) The alloyed hot-dip galvanized steel sheet having post-forming strength-increasing heat treatment performance as described in (3) or (4) above, wherein the microstructure is mainly composed of ferrite and the balance is made of pearlite and / or bainite.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The researchers are diligent about steel plates, heat treatment methods, and formability (particularly press formability) as a method of imparting deformation strength characteristics to members and parts while ensuring processability such as press formability of members and parts. When research was conducted, a steel sheet containing Cr, W, and Mo, which are carbide-forming elements having a weak affinity for C, was processed by a press forming method giving a strain of 2% or more and subjected to heat treatment at 200 to 450 ° C., It has been found that a molded body made of an alloyed hot-dip galvanized steel sheet is markedly cured without impairing corrosion resistance. Furthermore, it has been found that the amount of hardening increases by adding Ti, Nb, and V as steel compositions.
[0017]
The present invention is described in detail below.
[0018]
First, the reason for limiting the steel components will be described below.
[0019]
C is an element that affects the workability of steel, and the workability deteriorates as the content increases. Therefore, it is 0.08% or less. If it is less than 0.01%, the amount precipitated as carbide during heat treatment at 200 ° C. to 450 ° C. is small, and the effect of increasing the strength during heat treatment is small, so 0.01% is made the lower limit.
[0020]
If Si is less than 0.005%, the manufacturing cost will increase drastically, making it less economical, so 0.005% is the lower limit, and if it exceeds 1.0%, the workability deteriorates and zinc does not adhere easily. Since the adhesion of galvanizing is impaired, 1.0% is made the upper limit.
[0021]
If Mn is less than 0.01%, the manufacturing cost will be drastically increased and it will not be economical. Therefore, 0.01% is the lower limit, and if it exceeds 3.00%, the workability deteriorates. The upper limit.
[0022]
P is an element that can increase the strength without impairing the deep drawability, and is added according to the strength level. However, if it is less than 0.001%, the manufacturing cost is drastically increased and it is not economical. 0.001% is the lower limit, and if it exceeds 0.15%, secondary work brittleness will occur, so 0.15% is the upper limit.
[0023]
S is essentially an element that is meaningless to be present in steel, so it is better to be less, but if it is less than 0.001%, the manufacturing cost will increase dramatically and it will not be economical, so 0.001% is the lower limit. If it exceeds 0.02%, red hot embrittlement occurs during hot rolling, and cracks occur on the surface, so-called hot brittleness, so 0.02% is made the upper limit.
[0024]
Al is usually added as a deoxidizing component, and in order to prevent the occurrence of defects such as blow holes, it is necessary to add 0.001% or more. If it is less than 0.001%, it cannot be sufficiently deoxidized, so 0.001% is made the lower limit. Further, if it exceeds 0.1%, the effect of deoxidation is saturated, so 0.1% is made the upper limit. Further, in order to control the form of inclusions by further adding Ti to reduce press cracks, Al is preferably 0.001 to 0.005%.
[0025]
N is preferable to be small in order to ensure workability, but if it is less than 0.0002%, the manufacturing cost increases dramatically and is not economical, so 0.0002% is the lower limit and exceeds 0.01%. Since the workability deteriorates, 0.01% is made the upper limit.
[0026]
In order to exhibit the effect of increasing the tensile strength during the predetermined low-temperature heat treatment after work forming, in addition to the above elements, one or more of Mo, W, and Cr are further added as essential elements. The total amount of W, Cr is 0.05 to 3.0% in steel. If the total amount is less than 0.05%, the tensile strength cannot be sufficiently increased even if the heat treatment is performed. If the total amount exceeds 3.0%, Mo, W, and Cr are steel strengthening elements, the strength before work forming becomes too high, and the workability is impaired. This is because the effect of increasing the thickness is small or saturated, which is economically inconvenient.
[0027]
When the steel sheet containing Cr, W, and Mo is subjected to work forming such as press forming that imparts a strain of 2% or more, and subjected to heat treatment at over 200 ° C. to 450 ° C., the tensile strength of the steel plate increases. The reason for the increase in tensile strength when a predetermined amount of Cr, W, Mo is added to the steel and this heat treatment is not clear, but by applying a strain of 2% or more, a considerable amount is added to the ferrite part in the steel plate. Because dislocations are introduced, the diffusion of C, Cr, W, and Mo in steel increases dramatically even at low temperatures, and further, precipitation of Cr, W, and Mo carbides occurs on the dislocations using the introduced dislocations as nuclei. The present inventors consider that carbide precipitates in a short time at a low temperature and the tensile strength of members and parts increases.
[0028]
Further, as will be described later, this effect is remarkable when the microstructure is ferrite, when it is mainly composed of ferrite (particularly when the ferrite structure fraction is 60% or more), or when it is composed mainly of ferrite and the remainder is pearlite and / or bainite. is there.
[0029]
Further, in order to increase the tensile strength at the time of heat treatment after work forming, Ti, Nb and V can be added as selective elements in addition to the Cr, W and Mo. The reason why the effect of increasing the tensile strength and hardness of a member or part containing Ti, Nb, V is not clear, but when the present inventors add Ti, Nb, V, the above Cr, W It exerts a synergistic effect with the action of low-temperature heat treatment due to the addition of Mo, and forms fine carbides in the steel sheet. This fine carbide effectively proliferates dislocations against the strain applied during pressing, and the amount of strain This is because the effect of increasing the number appears. In particular, addition of V and addition of one or two of Ti and Nb are preferred because the heat treatment after work forming can be shortened.
[0030]
Ti is an element that enhances the effect of increasing the strength during heat treatment. If less than 0.005%, the effect is small, so 0.005% is made the lower limit. Ti is an element that increases the strength of the steel, and if it exceeds 0.1%, workability deteriorates, so 0.1% is made the upper limit.
[0031]
Nb is an element that enhances the effect of increasing the strength during heat treatment, and if less than 0.005%, the effect is small, so 0.005% is made the lower limit. Nb is an element that increases the strength of steel. If it exceeds 0.1%, workability deteriorates, so 0.1% is made the upper limit.
[0032]
V is an element that enhances the effect of increasing the strength during heat treatment, and if less than 0.005%, the effect is small, so 0.005% is made the lower limit. V is an element that increases the strength of the steel. If it exceeds 0.1%, workability deteriorates, so 0.1% is made the upper limit.
[0033]
Although the components are adjusted as described above, in order to increase the effect of increasing the strength during the heat treatment after forming, it is desirable to keep the amount of C in the steel sheet in a solid solution state at the heat treatment temperature. The elements Ti, Nb, and V are less than {(48/12) × C [%] + (48/14) × N [%]}} in terms of Ti, or the amount of Nb is {(93/12) × C [ %] + (93/14) × N [%]} or less, or the V amount is {(51 × 4/12/3) × C [%] + (51/14) × N [%]} or less, or When Ti, Nb, and V are added together, {Ti [%] × 12/48 + Nb [%] × 12/93 + V [%] × 12 × 3/51/4} <C [%] + N [%] × It is desirable to add so as to satisfy 12/14.
[0034]
Next, the reason for limiting the microstructure of steel will be explained.
[0035]
The microstructure of the steel is made of ferrite or is mainly composed of ferrite (the fraction of ferrite in the microstructure of the steel is 60% or more). Ferrite is soft and excellent in workability, and can accumulate many dislocations in crystal grains when processed. Further, when strain is applied, dislocations enter uniformly, and the strength of the entire steel sheet can be increased uniformly. Therefore, the microstructure of the steel is made of ferrite or mainly composed of ferrite. Furthermore, if the remaining structure of the ferrite-based structure that is not ferrite is pearlite and / or bainite, stress concentration is increased at the interface between the ferrite and the remaining structure, and dislocations can be effectively imparted to the ferrite. In addition, since the pearlite and / or bainite structure is not as hard as the martensite structure, the pearlite and / or bainite structure itself is deformed and the amount of dislocation increases as a whole of the steel sheet, so that the strength of the steel is effectively increased during the heat treatment.
[0036]
The post-molding strength increasing heat treatment of the present invention is a process in which a strain equivalent to 2% or more is applied to a plastic equivalent strain, and then in a temperature range of 200 to 450 ° C. (more preferably 220 to 370 ° C.) With tensile strength ΔTS (= TS after thermomechanical treatment − TS before processing) that compares the tensile strength before and after processing in a short period of 30 minutes (heating after cold forming or temperature holding after warm forming, etc.) Heat treatment capable of improving strength by 60 MPa or more (more preferably 90 Mp or more by ΔTS) or increasing by Vickers hardness ΔHv (= Hv after work heat treatment−Hv before work) by 18 or more (more preferably by 27 or more by ΔHv) Indicates. However, this heat treatment does not require a hardening element (for example, nitrogen or the like) to actively enter the molded body from the outside like a nitriding process. The high strength alloyed hot-dip galvanized steel sheet as a heat treatment material for increasing strength (tensile strength) after work forming of the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, and the thickness is not limited. Is particularly effective at 0.4 to 6 mm.
[0037]
In the production of the steel of the present invention, the molten steel adjusted to the above components is formed into a slab or steel slab by a continuous casting method or a steel slab by a granulation method, and heated without heating or after heating. Apply hot rolling. After hot rolling, descaling is performed, hot dip galvanizing is performed as it is, and then heat alloying is performed to obtain a high-strength galvannealed steel sheet. There are no particular restrictions on the hot rolling and winding conditions, and it is carried out in accordance with conventional methods.
[0038]
Or after a hot rolling, a descaling process is performed and it cold-rolls to make a cold-rolled steel sheet. Then, annealing and hot dip galvanizing are performed, and then heat alloying treatment is performed to obtain a high-strength galvannealed steel sheet. The annealing temperature at this time is sufficient if it is a normal temperature. Further, the heating method of the heat alloying treatment is not particularly limited, and direct heating by combustion gas, induction heating, direct current heating, or the like can be appropriately selected.
[0039]
Steel sheet that has been temper-rolled for improved workability and appearance after processing after being formed into an alloyed hot-dip galvanized steel sheet. Steel plate, etc.), any steel plate that has been subjected to surface treatment that is normally used as a thin steel plate, such as a steel plate having an oil film layer such as rust preventive oil, lubricating oil, etc., as long as it is a steel plate within the component range of the present invention. The effects of the present invention can be fully enjoyed.
[0040]
Next, press working such as drawing is performed using the steel plate having the above chemical components. In performing the press working, in order to give an appropriate amount of dislocation to the steel sheet, forming that applies a plastic equivalent strain of 2% or more is performed on a portion where strength (tensile strength) and hardness are required. If the amount of strain is too small, the effect of increasing the strength of the present invention may not be sufficiently exhibited even after heat treatment after molding. Therefore, the amount of strain applied during pressing is preferably 2% or more, more preferably 5%. That's it. The press molding method is not particularly limited as long as it gives a strain of 2% or more, and there is no problem even if drawing, overhanging, bending, ironing, punching, or the like is added. FIG. 2 shows the relationship between the amount of strain during press molding and the amount of increase in tensile strength (ΔTS) after press molding and heat treatment after molding. It can be seen that the increase in tensile strength is significant at strains of 2% or more, preferably 5% or more.
[0041]
After press molding, heat treatment is performed at a low temperature. At this time, if the heat treatment temperature is less than 200 ° C., the effect of the present invention cannot be exhibited, so 200 ° C. is set as the lower limit. If the temperature exceeds 450 ° C, the iron-zinc alloying reaction of the galvanized layer proceeds, the iron concentration in the plated layer increases abnormally or it becomes difficult to obtain the desired alloy phase distribution, and the corrosion resistance of the finished member or part Therefore, the temperature is set to 450 ° C. or lower. The heat treatment method for heating to a temperature of 200 to 450 ° C. is not particularly specified, and at least the strain imparting part is set to a temperature of 200 to 450 ° C., such as partial high-frequency heating, current heating, warm bath heat treatment, infrared heating, hot air heating, and the like. Any heating method may be used. FIG. 3 shows the relationship between the heat treatment temperature and the amount of increase in tensile strength (ΔTS) after heat treatment. The temperature range of the post-molding heat treatment in which ΔTS after molding is more preferably 90 MPa or more is preferable. It is the range of -370 degreeC.
[0042]
【Example】
The present invention will be specifically described below based on examples.
Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. And it heated to 1200 degreeC in the heating furnace, hot-rolled with the finishing temperature of 880 degreeC, wound up at 550 degreeC, and then pickled, and it was set as the hot-rolled steel plate.
[0043]
Further, a part of the hot-rolled steel sheet was further subjected to cold rolling at a reduction rate of 80%, and then subjected to recrystallization annealing at 830 ° C. for 60 seconds to obtain a cold-rolled steel sheet.
[0044]
The obtained hot-rolled steel sheet and cold-rolled steel sheet were processed into JIS No. 5 tensile test pieces, and mechanical property values (no heat treatment) were evaluated.
[0045]
Separately, the steel sheet was formed by pressing to form a hat-shaped press-formed product shown in FIG. At this time, the wrinkle pressure was adjusted, and an average plastic strain of 5% was applied to the vertical wall portion A and 2% to the flat portion B. The parts were placed in a furnace maintained at 250 ° C. for 10 minutes, then air cooled and heated. Tensile test pieces were cut out from the vertical wall part A and flat part B of the part, and the tensile strength was measured. In the tensile test after pressing, the true stress-strain relationship is measured, so in order to see the increase in nominal stress, the plate thickness before pressing is converted to the test piece thickness. The nominal stress.
[0046]
The above results are also shown in Table 1.
[0047]
As is apparent from Table 1, the steel sheet of the present invention is excellent in heat treatment curability.
[0048]
[Table 1]

[0049]
【The invention's effect】
The alloyed hot-dip galvanized steel sheet of the present invention is formed by press forming or the like using a relatively soft high strength steel having a strength level of 300 Mpa class, 400 Mpa class, 500 Mpa class, 600 Mpa class, or 700 Mpa class before processing. It is easy, and after forming processing such as press molding, heat treatment at a relatively low temperature is performed for a short time to increase the tensile strength or hardness, thereby increasing the deformation strength of members and parts, or increasing the rigidity. Since it is a possible steel sheet, it is applied to places where structural strength, particularly strength and / or rigidity at the time of deformation is required, such as construction members, machine structural parts, and automobile structural parts.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating stress-strain curves of a steel sheet of the present invention and a conventional steel sheet.
FIG. 2 is a graph showing the relationship between the amount of strain imparted to the steel sheet of the present invention by press forming and the amount of increase in tensile strength during post-forming heat treatment.
FIG. 3 is a graph showing the relationship between the heat treatment temperature after molding and the amount of increase in tensile strength after heat treatment.
FIG. 4 is a schematic diagram showing the shape of a hat-type press-formed product.
[Explanation of symbols]
A Wall B Flat part

Claims (5)

C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or more of W, Cr and Mo in a total amount of 0.05 to 3.0%, the balance being iron and inevitable impurities, and the microstructure of the steel is ferrite An alloyed hot-dip galvanized steel sheet having post-forming strength increasing heat treatment performance, characterized by comprising: As a steel composition, the weight percentage
Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%,
V: 0.005 to 0.1%,
The alloyed hot-dip galvanized steel sheet having post-molding strength-increasing heat treatment performance according to claim 1, wherein one or more of these are contained. C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or more of W, Cr, and Mo, the total amount is 0.05 to 3.0% ,
Ti: 0.005 to 0.1%,
Nb: 0.005 to 0.1%,
V: 0.005 to 0.1%,
An alloy having a heat-treating property for increasing strength after forming, characterized in that the steel contains one or more of the following, the balance being iron and inevitable impurities, and the microstructure of the steel is mainly composed of ferrite Hot dip galvanized steel sheet. The alloyed hot-dip galvanized steel sheet having post-forming strength-increasing heat treatment performance according to claim 3 , wherein the ferrite has a structure fraction of 60% or more. 5. The alloyed hot-dip galvanized steel sheet having post-forming strength increasing heat treatment performance according to claim 3 or 4 , wherein the microstructure is mainly composed of ferrite and the balance is made of pearlite and / or bainite.

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